Antimony oxide glass with optical activity

ABSTRACT

According to one aspect of the present invention an optically active glass contains Sb 2 O 3 , up to about 4 mole % of an oxide of a rare earth element, and 0-20 mole % of a metal halide selected from the group consisting of a metal fluoride, a metal bromide, a metal chloride, and mixtures thereof, wherein this metal is a trivalent metal, a divalent metal, a monovalent metal, and mixtures thereof. In addition, any of the glass compositions described herein may contain up to 15 mole % B 2 O 3  substituted for an equivalent amount of Sb 2 O 3 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No.09/288,454 filed on Apr. 4, 1999 now U.S. Pat. No. 6,410,467, whichclaims the benefit of U.S. Provisional Patent Application No.60/081,073, filed Apr. 8, 1998; which the content of which is reliedupon and incorporated herein by reference in its entirety, and thebenefit of priority under 35 U.S.C. § 120 is hereby claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to antimony oxide-containingglass compositions and, more particularly, to optically active antimonyoxide-containing glasses that are made optically active by being dopedwith a rare earth element; their use in optical amplifying devices andoptical amplifying devices incorporating these compositions; and methodsfor making the glass compositions of the invention. As used herein, theterm “optically active” refers to a rare earth doped glass capable ofstimulated emission for amplifying a light signal when the glass isexcited by a suitable pumping source.

2. Technical Background

It is known that single mode optical fibers doped with low levels ofrare-earth ions can be drawn from heavy metal fluoride glasses.Unfortunately, heavy metal fluoride glasses suffer certain undesirableattributes that have restricted their applications. Most notably, heavymetal fluoride glasses exhibit poor resistance to devitrification. U.S.Pat. No. 4,674,835 discusses the crystallization problems of heavy metalfluoride glasses, one example of which is referred to as ZBLAN, and thelight scattering problems resulting therefrom. The susceptibility ofheavy metal fluoride glasses to devitrification generates problems inmaking large preforms. Crystallization in the preform causesdifficulties during the formation of optical fibers by commonly usedmethods. Heavy metal fluoride glasses are quite prone to inhomogeneousnucleation, which leads to crystallization at the core and claddinginterfaces during the drawing of the optical fiber. The resultingcrystals in the fibers cause serious light scattering losses.

Devitrification of the heavy metal fluoride glasses is aggravated whenions necessary to impart differences in indices of refraction to thecore and cladding are added to the glass composition. Additional doping,for example, with rare earth metal ions, also tends to reduce thestability of the glass. As a consequence of those problems, research hasfocused on finding additives to a base fluoride glass composition thatwill reduce the tendency of the glass to devitrify and to increase thechemical stability thereof. However, the preparation of fluoride glassesrequires that the glass forming components be reheated at hightemperatures. Furthermore, these glasses cannot be melted in air butrequire a water-free, inert gas environment.

Most oxide glasses such as, for example, silicon dioxide, are easier toprepare, more chemically and mechanically stable, and more easilyfabricated into rods, optical fibers, or planar waveguides than arefluoride glasses. Addition of even small amounts of oxides into fluorideglasses to improve their stability significantly quenches theirupconversion luminescence.

Rare earth-doped glasses have found frequent use for the fabrication oflight-generating and light-amplifying devices. For example, U.S. Pat.No. 3,729,690 describes a laserable glass comprising a host materialthat contains a fluorescent trivalent neodymium ingredient. U.S. Pat.No. 5,027,079 discloses an optical amplifier comprising a single modefiber that has an erbium-doped core. U.S. Pat. No. 5,239,602 disclosesan apparatus and method for flattening the gain of an optical amplifierthat utilizes an erbium-doped silica fiber having a germanosilicatecore. U.S. Pat. No. 5,563,979 discloses an erbium-doped planar opticaldevice whose active core includes a mixture of oxides such as lanthanumand aluminum oxides. The inclusion of antimony oxide in glasses foroptical devices is also reported. One reference describes a glass foruse in waveguides that contains 50-75 mole % Sb₂O₃.

For the construction of efficient optical amplifiers, there remains aneed for new, readily prepared glasses that display an optimalcombination of gain flatness and breadth. This need is well met by theglass of the present invention.

SUMMARY OF THE INVENTION

According to one aspect of the present invention an optically activeglass contains Sb₂O₃, up to about 4 mole % of an oxide of a rare earthelement, and 0-20 mole % of a metal halide selected from the groupconsisting of a metal fluoride, a metal bromide, a metal chloride, andmixtures thereof, wherein this metal is a trivalent metal, a divalentmetal, a monovalent metal, and mixtures thereof. In addition, any of theglass compositions described herein may contain up to 10 mole % B₂O₃substituted for an equivalent amount of Sb₂O₃.

Although the glass of the present invention is highly desirable becauseit can be fabricated in air using standard melting techniques and batchreagents, when the glass contains about 90% or more of Sb₂O₃ it isformed by the techniques of splat quenching or roller quenching. Theglass composition of the present invention exhibits a gain spectrum withexcellent breadth and flatness characteristics and can be readilymodified for specific optical amplifier applications.

Further in accordance with the present invention is an optical energyproducing or light-amplifying device, in particular an optical amplifieror laser, that comprises the glass of the invention. The opticalamplifier can be either a fiber amplifier or a planar amplifier, eitherof which may be of a hybrid (composition) construction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graph comparing the emission spectra from 1400 nm to 1700nm of an aluminosilicate glass, a fluoride glass (ZBLAN), and anerbium-doped, antimony-containing glass of the invention;

FIG. 1B is a detailed version of FIG. 1A over the range 1500 nm to 1600nm;

FIG. 2 is a plot of the calculated gain spectra for a glass of theinvention; and

FIG. 3 is a graph of the calculated gain spectra for 61-65% inversion in0.5% steps for a glass of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The optically active glass of one embodiment of the present invention,expressed in mole percent on an oxide basis, comprises Sb₂O₃ and up toabout 4 mole % of an oxide of a rare earth element. The glass preferablycomprises 0.5-99 mole % Sb₂O₃, and preferably about 0.01-0.4 andpreferably 0.05 to 0.2 mole % of rare earth element oxide, such as,Er₂O₃ for example. The glass preferably comprises a remainder of one ormore compatible metal oxides.

In one embodiment, the optically active glass consists essentially ofSb₂O₃ and up to about 4 mole % RE₂O₃, where RE is an oxide of a rareearth element. Although erbium is the especially preferred rare earthelement, the glass may comprise other rare earth elements to impartoptical activity to the glass as defined herein, as further describedbelow.

It will be appreciated by those skilled in the art that the rare earthelement plays no part in forming the glass per se. Thus an embodiment ofthe invention is a glass consisting essentially of Sb₂O₃.

The glass of the invention can further comprise 0-99 mole % SiO₂, 0-99mole % GeO₂, and 0-75 mole % Al₂O₃ or Ga₂O₃.

In an aspect of each of the embodiments of the invention, up to 15 mole% B₂O₃ can be substituted for an equivalent amount of Sb₂O₃. While B₂O₃reduces the lifetime of erbium envision at 1530 nm, its advantage isthat it reduces the lifetime of the 980 pumping level (τ₃₂) at a fasterrate.

The glass of the invention can further comprise 0-45 mole % A₂O, where Ais Li, Na, K, Rb, Cs, or mixtures thereof, and/or 0-45 mole % MO, whereM is Mg, Ca, Sr, Zn, Ba, Pb, or mixtures thereof:. The rare earthelement is Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,Lu, or mixtures thereof, and Scandium (Sc) may be substituted for a rareearth in an embodiment of the invention. In one embodiment of thepresent invention, the glass comprises 50-72 mole % SiO₂, 0-20 andpreferably 0-6 mole % Al₂O₃, 2-35 and preferably 10 to 30 mole % Sb₂O₃,0-5 mole % K₂O, and 0.01-0.2 mole % Er₂O₃.

In another embodiment of the present invention, the glass comprises10-95 mole % SiO₂, 0-30 mole % Al₂O₃, 3-90 mole % Sb₂O₃, and 0.01-2 mole% Er₂O₃.

In another embodiment of the present invention, the glass furthercomprises a metal fluoride, bromide, chloride, or mixtures thereof. Themetal fluoride can be incorporated for the purpose of insuring that theglass has low water content. However, it should be stated that dry glasswith and glass with optimal erbium spectroscopy can be produced withoutincorporation of metal halides. The metal forming the metal halide canbe a trivalent, divalent, or monovalent metal, or mixtures thereof. In afurther preferred embodiment, the metal halide is a metal fluoride suchas AlF₃, CaF₂, KF, or mixtures thereof. The (metal fluoride)/(metalfluoride+total oxides) molar fraction of the glass is preferably about0.01 to 0.25, more preferably about 0.1 to 0.2.

In yet another embodiment of the present invention, the glass comprises50-72 mole % SiO₂, 0-20 mole % and preferably 0-6 mole % Al₂O₃, 2-35mole % Sb₂O₃, 0-20 mole % and preferably 0-5 mole % K₂O, and about 0.1mole % Er₂O₃, and further includes 0.1-20 mole % of a metal halide.

Further in accordance with the present invention is an opticalenergy-producing or -amplifying device. Preferably, the device is anoptical amplifier comprising the rare earth element-doped, antimonyoxide containing glass described above. The optical amplifier can beeither a fiber amplifier or a planar amplifier, as described in, forexample, U.S. Pat. Nos. 5,027,079, 5,239,607, and 5,563,979, thedisclosures of which are incorporated herein by reference. The fiberamplifier can further be of a hybrid structure that combines legs formedfrom a glass of the invention with legs formed from a standardaluminosilicate glass, as described, for example, in M. Yamada et al.,“Flattening the gain spectrum of an erbium-doped fiber amplifier byconnecting an Er³⁺-doped SiO₂—Al₂O₃ fiber and an Er³⁺-dopedmulticomponent fiber,” Electronics Lett., 30, pp. 1762-1764 (1994), thedisclosure of which is incorporated herein by reference.

As discussed in the commonly-assigned, previously-filed, co-pending U.S.Pat. application Ser. No. 60/081,073, filed on Apr. 8, 1998, thedisclosure of which is incorporated herein by reference, the localbonding environments of rare earth elements (“REE”) in glasses determinethe characteristics of their emission and absorption spectra. Severalfactors influence the width, shape, and absolute energy of emission andabsorption bands, including the identity of the anion(s) andnext-nearest-neighbor cations, the symmetry of any particular site, thetotal range of site compositions and symmetries throughout the bulksample, and the extent to which emission at a particular wavelength iscoupled to phonon modes within the sample. Fluoride glasses are usefulhosts for optically active REE, because the fluorine atoms surroundingthe REEs substantially impact REE emission and absorption spectra. Theextreme electronegativity of fluorine lifts the degeneracy of theelectronic states of the REE, producing emission and absorption bandsthat differ substantially from those produced in oxide hosts, beingbroader and having different relative intensities and, sometimes,different positions. The emission bands are often blue-shifted relativeto their positions in oxide glasses. In general, the absolute positionand width of an emission or absorption band shifts to lower energy asthe electronegativity of the surrounding anions decreases: for example,the total bandwidth of the Er³⁺ 1530 nm emission band in fluorideglasses, such as ZBLAN, is greater than in nearly any oxide glass, andthe high-energy edge of the emission band in a fluoride glass is at ahigher energy than in an oxide glass. In certain systems, such itshybrid oxyfluoride glasses, it is possible to obtain much of thebandwidth and gain flatness of a fluoride glass by creating environmentsfor the REE that are a combination of oxide and fluoride-like sites.

For optical amplifier applications, the region over which a convolutionof the emission and absorption is the flattest is the optimal windowthrough which to pass signals. Because both the position of the overallemission bands and the structure within the band vary from fluoride tooxide hosts, the window with optimal gain flatness also varies. Ideally,one would like to obtain the broadest emission possible in a singleglass.

Relative to oxide glasses, fluoride glasses also can accommodate veryhigh concentrations of REE without incurring nonradiative lossesresulting from energy transfer between the REE. However, fluorideglasses are prepared under controlled atmospheres; they have extremelyhigh coefficients of thermal expansion and are environmentally unstablecompared to many oxide glasses, which complicates their use in practicalapplications. It is desirable to have glasses with bonding environmentsfor rare earth elements that produce spectoscopic attributes comparableto, or improving upon fluoride glass hosts, while retaining the physicaland chemical characteristics of oxide glasses.

As noted above, glasses having broad, flat emission spectra are highlydesirable for optical amplifier applications. A flat emission spectrumis defined as one having less than 10% gain ripple over bands (orwindows) up to 38 nm wide. Inclusion of fluorine in a glass results inimproved dispersal of the REE throughout the glass, which facilitateshigher REE loadings without degradation of lifetime. Higherconcentrations of REE that are dispersed in separate locations and arethus unable to physically interact with each other are believedpossible. The REE include Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, and Lu. It is noted that scandium may be substituted for arare earth element. In accordance with one aspect of the presentinvention, Er is especially preferred.

The REE ions are highly soluble in the glasses of the present invention,which allows higher loadings of REE. Accordingly, the use of the glassof the invention makes it possible to reduce the size of an opticalamplifier because less waveguide material for the same amount of gain isrequired. Furthermore, because the glass of the present invention canprovide quantum efficiency upon radiation substantially equal to 100%,less powerful pump lasers are required to generate fluorescent emission.Useful fluorescent emission maxima are in the range of about 1.3 μm toabout 1.8 μm. Fluorescent emission maxima of Er-doped glasses aretypically in the range of about 1.5μm to about 1.6μm.

As is well known in the art, Er-doped amplifiers are typically pumped inthe 980nm or 1480nm wavelength band. In a preferred aspect of an opticalamplifier embodiment of the invention for signal amplification in the1500 nm telecommunications window (C-band) and/or in the extended erbiumspectrum from about 1565-1610nm (L-band), pumped at 980nm, up to 15 mole% B₂O₃ (and preferably 0 to 8 mole %) is substituted for an equivalentamount of Sb₂O₃. As stated above, the B₂O₃ reduces the τ₃₂ lifetime,which is advantageous for pumping an erbium-doped optical amplifier madefrom the glass compositions of the invention at 980nm. In a furtherpreferred aspect of an optical amplifier embodiment of the invention, upto 15 mole % As₂O₃ (arsenic-trioxide), up to 15 mole % Tl₂O (thalliumoxide), up to 15 mole % In₂O₃ (indium oxide), and up to 15 mole % Bi₂O₃(bismuth trioxide), can be included in the compositions of the inventionto modify physical properties such as refractive index and viscositywith no adverse effect on amplifier performance.

Substitutions of germanium and lead for silicon or gallium for aluminumcan be used to modify glass forming temperatures, viscosity curves,expansivity, and refractive index. Alkali and alkaline earth metals canbe included in the glass to vary the refractive index and to increase ordecrease thermal expansivity. Glasses containing optically active REEcan be co-doped with non-active REE (for example, Er co-doped with La orY) co-doped with optically active REE (such as Er co-doped with Yb) toimprove quantum efficiency. By varying bulk composition, glasses can beformed with variable optical properties, thus affording maximumflexibility.

The glass of the present invention has absorption and emissioncharacteristics that are effectively hybrids of the best characteristicsobtained in oxide or fluoride glasses alone. However, unlike fluorideglasses, which must be prepared in an inert atmosphere, embodiments ofthe glass of the present invention can be fabricated in air usingstandard melting techniques and batch reagents. In addition, theenvironmental stability of the hybrid glasses considerably exceeds thatof pure fluoride glasses.

The properties of the glass of the present invention make itparticularly useful for the fabrication of a variety of optical devices.Provided with a compatible covering or cladding, the glass can be formedinto fiber or planar optical amplifiers or lasers. It can be used alonein planar amplifier applications, or in combination with clad glassesfor double-crucible fiberization or rod-and-tube redraw. In addition,the emission/absorption spectra of glasses prepared in accordance withthe invention may be tailored to “fill in holes” in the gain spectrum ofconventional amplifier materials such as silica or ZBLAN, for example,resulting in hybrid amplifiers that provide a greater degree of gainflatness than can be obtained from any of these materials alone. Inaddition, glasses free of rare earth elements can be prepared withmechanical and viscoelastic properties very similar to those of glassesdoped with rare earth elements, but lower refractive indices. Theserefractive index-adjusted glasses can serve as clad glasses foroptically active cores such as in a fiber laser or in a fiber amplifier.

Embodiments of the glass of the invention can generally be producedaccording to standard techniques for making glasses: providingglass-forming components and treating these components under conditionseffective to produce the glass, which generally entails melting theglass-forming components to produce a glass melt, forming the glass meltinto a shaped article, which is then cooled. Preferably, the componentsare melted in air at a temperature of about 1300-1700° C. for about 2hours to about 20 hours to produce the glass melt. Next, the glass meltis formed into a shaped article by forming procedures such as, forexample, rolling, pressing, casting, or fiber drawing. A shaped articlesuch as, for example, a patty, rod, or sheet, is cooled and thenannealed at a temperature of about 350-450° C. for about 0.5 hour to 2hours. After the final heat treatment, the shaped article is allowed tocool to room temperature.

Certain embodiments of the glass compositions of the present invention,namely those including about 90 mole % or more Sb₂O₃, were prepared bysplat quenching or roller quenching. Since antimony is not compatiblewith platinum, the high content antimony oxide glasses of the inventionare melted in silica or alumina crucibles. During heat up, some of theSb₂O₃ changes to Sb₂O₅, and on cooling forms the very refractorycrystalline phase cervantite, Sb₂O₄. This problem is alleviated by splatand/or roller quenching as described in Examples 1-3, below. A possiblealternative is to melt Sb₂O₃ in a dry box, or in sealed silica ampules,known to those skilled in the glass forming art.

Table I lists some preferred exemplary compositional embodiments of theinvention. The numerical values are in moles %.

TABLE I Sb₂O₃ SiO₂ GeO₂ Al₂O₃ B₂O₃ Ga₂O₃ Cs₂O In₂O₃ Na₂O K₂O F RE₂O₃ 909.9 0.1 90 94.9 9.9 0.1 5 94.9 0.1 5 0.1 35 25 38 2 35 25 38 2 75 24.90.1 30 69.9 0.1 99.9 0.1 30.3 60.6 3.03 1.52 1.52 1.52 1.52 1 1 27.7755.54 4.63 4.63 4.63 1.4 1.4 1 1 35 50 0 15 0 0.1 15.5 74 4 5 1.5 0.1

EXAMPLES

The following examples further illustrate the preparation ofErbium-Doped, Antimony Oxide-Containing Glass.

Example 1

The following composition

Sb₂O₃ 99.9 mole % Er₂O₃  0.1 mole %

was prepared as follows: A 25g charge of melt was held at 25-50° C.above its liquidus in a silica crucible until it reached thermalequilibrium, about 10-15 min.

In a preferred method aspect for forming this glass by splat quenching,the charge is delivered to a cold plate (e.g., steel or graphite) andsmashed from above by a cold “hammer” (e.g., steel or graphite). With agood configuration, the quench rate is ≧250° C./sec.

In another preferred method aspect for forming this glass by rollerquenching, the charge is delivered between cold rollers (e.g., steel orgraphite). Depending upon the thermal conductivity of the sample, thequench rate is >>1000° C./s.

Larger melt samples of the glass can be similarly processed, but thelateral dispersal of the melt in the splat quench limits the largestsize that can be handled to about 150 g. The glass in a roller quenchoperation is delivered as a continuous stream, thus there is no sizelimit.

Example 2

The following composition

Sb₂O₃ 90.0 mole % SiO₂  9.9 mole % Er₂O₃  0.1 mole %

was prepared by splat quenching as described in Example 1, above.

Example 3

The following composition

Sb₂O₃ 99.0 mole % GeO₂  9.9 mole % Er₂O₃  0.1 mole %

was prepared by splat quenching as described in Example 1, above.

Example 4

A glass-forming mixture having the following composition (in mole %) isball-milled and charged into a silica crucible:

SiO₂ 55 Al₂O₃ 10.4 Al₂F₆ 5 K₂O 0.6 K₂F₂ 10.5 K₂Br₂* 1.5 Sb₂O₃ 17 Er₂O₃0.1 *added to eliminate water from final glass

The crucible is covered and at a temperature of about 1425° C. for about2 hours. The melt is poured onto a steel plate to form a sheet, which iscooled, then placed in an annealing oven and held at a temperature ofabout 450° C. for about one hour before being allowed to cool graduallyto room temperature.

Example 5

A glass-forming mixture having the following composition (in mole %) isball-milled and charged into a silica crucible:

SiO₂ 75.5 Al₂F₆ 2 Sb₂O₃ 22.5 Er₂O₃ 0.1 Fluoride 2 wt %

The crucible is covered and heated at a temperature of about 1475° C.for about 6 hours. The melt is poured onto a steel plate to form asheet, which is cooled, then placed in an annealing oven and held at atemperature of about 450° C. for about one hour before being allowed tocool gradually to room temperature.

Example 6

A glass-forming mixture having the following composition (in mole %) isball-milled and charged into a silica crucible:

SiO₂ 75.5 Al₂O₃ 2 Sb₂O₃ 22.5 Er₂O₃ 0.1

The glass is melted at a temperature of 1525° in a silica crucible.After pouring, the glass is then annealed at a temperature of 500° for 1hour before being allowed to cool gradually to room temperature. Theremoval of fluoride improves durability and resistance to derititicationand phase separation. This greatly diminishes scattering loss atequivalent fiber draw temperatures (1175° C.) from 1 dB/m with λ⁻⁴wavelength dependence at 2 % F to 0.076 dB/m with no wavelengthdependence at wt/0% F.

Spectroscopic Analysis of Glass Samples

The absorption spectra of polished 10×10×20-mm samples of the glassprepared as described in Example 4, an aluminosilicate glass(CaAl₂Si₂O₈), and a fluoride glass (ZBLAN) are measured using a Nicolet(Madison Wis.) FT-IR spectrophotometer with 4 cm⁻¹ resolution, 256 FID'sper sample being collected. The Er fluorescence emission spectra isgenerated by pumping the 520 nm absorption band with a Xenon lamp, andthe 1.5 μm emission is measured using a liquid nitrogen-cooled Sidetector together with a SPEX Fluorolog (Edison N.J.) spectrophotometer.Data are collected over the range 1400-1700 nm in 0.5 nm steps, 1.5seconds/step. Each spectrum is corrected by subtraction of thebackground, then normalized to a value of 1.0 for the maximum peakintensity.

The spectra so obtained for the three samples over the range 1400-1700nm are depicted in FIG. 1A; a detail for the range 1500-1600 nm is shownin FIG. 1B. The breadth of the spectrum of the glass of the presentinvention greatly exceeds that of the aluminosilicate glass and alsoexceeds that of ZBLAN in the peak region around 1530-1560 nm by about 7nm.

Determination of Gain Flatness for Erbium-Doped, AntimonyOxide-Containing Glass

For a sample of the glass prepared as described in Example 4, gainspectra are calculated, in steps of 10%, for levels of inversion rangingfrom zero to 100%. The plots of the resulting spectra are shown in FIG.2. Gain spectra are also calculated for inversion levels over the range61-65%, in steps of 0.5%. The percentages of inversion are calculatedassuming that the absolute absorption and emission intensity maxima areof equal magnitude. The resulting plots are shown in FIG. 3.

A figure of merit (FOM) of gain flatness is defined as (MAX−MIN)/MIN,where MAX and MIN are, respectively, the largest and smallest values forgain within a “window,” or specified wavelength range. For the glass ofExample 4, FOMs are calculated for “floating windows” of widths 30, 35,and 40 nm; the results are shown in Table II.

TABLE II Window width (nm) Wavelength range (nm) % Inversion FOM 301535-1565 63 7 35 1530-1565 63 7 40 1528-1568 63.5 14.5

As shown by the data of Table II, the calculated gain spectra show avery flat response (FOM=7, corresponding to a 7% gain ripple) for the 30and 35 nm windows, which is substantially maintained for a window of 38nm width. Even for the 40 nm-wide window, a desirably flat response(FOM=14.5, ca 15% gain ripple) is maintained. These excellent gainflatness results greatly exceed those attainable with previously knownsilica amplifier materials.

Although the invention has been described in detail for the purpose ofillustration, it is understood that such detail is solely for thatpurpose, and variations can be made therein by those skilled in the artwithout departing from the spirit and scope of the invention that isdefined by the following claims.

What is claimed:
 1. An optically active glass (in mole % on an oxidebasis), comprising: (i) equal to or greater than 5 mole % Sb₂O₃; (ii) upto about 4% of at least one of RE₂O₃, where RE is a rare earth element;(iii) greater than 0 to 20 mole % of a metal halide selected from thegroup consisting of a metal fluoride, a metal bromide, a metal chloride,and a mixture thereof, wherein said metal is a trivalent metal, adivalent metal, a monovalent metal, or a mixture thereof.
 2. The glassof claim 1, further comprising a remainder of a compatible metal oxide.3. The glass of claim 1 wherein Sb₂O₃ is equal to or greater than 5-99mole %.
 4. The glass of claim 1 further comprising: 0-99 mole % SiO_(2;)0-99 mole % GeO₂; and 0-75 mole % (Al₂O₃ or Ga₂O₃).


5. The glass of claim 4 further comprising: 0-75% A₂O, where A isselected from the group consisting of Li, Na, K, Rb, Cs, and mixturesthereof.
 6. The glass of claim 4, further comprising: 0-15% As₂O₃; 0-15%Tl₂O; 0-15% In₂O₃, and 0-15% Bi₂O₃.
 7. The glass according to claim 4further comprising: 0-45 mole % MO, where M is selected from the groupconsisting of Mg, Ca, Sr, Zn, Ba, Pb, and mixtures thereof.
 8. The glassaccording to claim 4 comprising: 10-95 mole % SiO₂, 0-30 mole % Al₂O₃,equal to or greater than 5-90 mole % Sb₂O₃, and about 0.01-0.2 mole %Er₂O₃.
 9. The glass according to claim 8 comprising: 50-72 mole % SiO₂,0-20 mole % Al₂O₃, equal to or greater than 5-35 mole % Sb₂O₃, and about0.01-0.2 mole % Er₂O₃ and further comprising 0-20 mole % K₂O.
 10. Theglass according to claim 8 wherein said glass contains 0 to 5 mole % ofmetal halide.
 11. The glass according to claim 10 wherein said metalhalide is a metal fluoride selected from the group consisting of AlF₃,CaF₂, KF, and mixtures thereof.
 12. The glass according to claim 4having a (metal fluoride)/(metal fluoride+total oxides) molar fractionof about 0.0 to 0.25.
 13. The glass according to claim 12 wherein saidfraction is about 0.0 to 0.05.
 14. The glass of claim 1 wherein 0-15mole % B₂O₃ is substituted for an equivalent amount of Sb₂O₃.
 15. Theglass according to claim 1 wherein the rare earth element is selectedfrom the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb, Lu, and mixtures thereof.
 16. The glass according toclaim 15 wherein the oxide of said rare earth element comprises Er₂O₃.17. The glass according to claim 16 further comprising about 0.01-0.4mole % Er₂O₃.